Device intended for mechanical separation of high-temperature sand present in a gas stream

ABSTRACT

A process for separation of solid particles of wide grain size distribution present in a turbulent gas stream includes, first, removal of the coarser particles in a separation chamber ( 1 ) with a high efficiency, then removal, with a high efficiency, of the remaining finer particles in a precipitation device ( 20 ). An assembly intended for separation of solid particles of wide grain size distribution present in a turbulent gas stream includes a separation chamber ( 1 ) intended to receive the solid particles in suspension and to remove from the stream the major part of the coarser particles, and a precipitation device ( 20 ) intended to separate the finer particles from the stream.

FIELD OF THE INVENTION

The present invention relates both to the improvements made to particleseparators and to separation of sand of wide grain size distributionresulting from a thermal regeneration process and present in ahigh-temperature gas stream.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,626,651 and international patent applicationWO-97/27,928 (see also British patent 2,264,655 published on May 24,1995) describe a certain number of forms of particle separators intendedfor separation of particles carried along in a turbulent gas stream.These disclosures are taken up here by way of reference.

In the description hereafter, the present invention is explained withinthe scope of the separation of sand particles present in a turbulenthigh-temperature gas flow and having a grain size distribution rangingbetween about 1 micron (lower limit) and about 100 microns (upperlimit). Such a particle-containing gas is a by-product obtained withinthe scope of foundry sand regeneration processes.

It is however clear that this illustration of the present inventionrelative to the separation of solid particles contained in a turbulentgas flow is not intended to limit the more wide-ranging scope of thepresent invention, the invention being also applicable to the separationof other solid particles suspended in a turbulent gas flow.

It has been discovered that sand grains of a size above about 30 micronstend to bounce on the collector plates of a precipitation device asdescribed in U.S. Pat. No. 5,626,651 or in the corresponding Europeanpatent EP-B1-626,880; this reduces the efficiency of their removal fromthe gas streams. It has also been observed that the intensity of thebounce of the sand grains on the collector plates and the resultingefficiency loss increase with the size of the grains. Removal of thesand fraction consisting of sand grains of a size above about 30 micronswould therefore require construction of precipitation devices withplates of very great length whose cost would be prohibitive because ofthe very large number of collector plates required.

The other problem connected with separation of large-size sand grains bymeans of plate precipitation devices lies in the tendency of this sandgrains to erode the collector plates, which leads to unjustifiedexpenses in order to replace the plates and concomitant interruption ofthe process of removing the sand present in the gas stream.

SUMMARY OF THE INVENTION

The object of the present invention is notably to overcome theaforementioned drawbacks by proposing a process and a set of particleseparators allowing to remove the sand grains of a size above about 30microns present in a gas stream before they enter another device, aprecipitation device for example. This objective is reached by using ameans specially designed for efficient separation of particles such assand grains of a size above about 30 microns present in a gas stream.

The object of the present invention is a process intended for separationof solid particles of wide grain size distribution present in aturbulent gas stream, first comprising removal of the coarser particlesin a separation chamber with a high efficiency. Separation is completedby removal, with a high efficiency, of the remaining finer particles ina suitable device, a plate precipitation or a porous-structure devicepierced with channels for example.

The present invention can thus be defined, on the one hand, as a processintended for separation of sand grains of wide grain size distributionpresent in a turbulent gas stream, first comprising removing the sandfraction consisting of large-size sand grains in a separation chamberprovided therefore, then removing the remaining fine sand grains in aplate precipitation device or an equivalent device (porous structurewith channels) whose length is much less than that required forefficient removal of the large grain size sand if the gas stream hadbeen directly fed into a separation device such as a plate precipitationdevice.

The stage of removal of the large-size sand grains carried along by theturbulent gas stream consists in passing the stream into the separationchamber so as to reduce the velocity of flow of said gas stream and tocause said particles to settle under the action of gravity over a shortdistance in the gas stream, then possibly to hit the back wall of saidchamber and to fall down along the wall, where the gas has a viscousflow at very low velocity, onto the bottom of said chamber.

The invention furthermore relates to the use of such a process intendedfor separation of sand grains carried along by a high-temperature gasstream, said stream coming from a thermal sand regeneration process.

The invention also relates to an assembly intended for separation ofsolid particles of wide grain size distribution present in a turbulentgas stream, comprising: (a) a separation chamber intended to receivesaid high-temperature turbulent gas stream containing said particles andto remove from said stream the major part of said particles of largersize, and (b) a precipitation device intended to receive the streamcoming from the separation chamber and to collect the finer particlespresent in the stream.

The separation chamber can comprise a horizontal line provided with aninlet port intended to receive the stream and another line intended fordischarge of said stream. connected to a horizontal slot in the backwall of the chamber extending over the total width of the chamber andsituated at the upper end of said chamber. The separation chamber alsocomprises lower parts which taper at the lower end, in which the largerparticles are collected after being removed from the gas stream.

The sand (larger grains) can be discharged into containers from saidchamber.

More precisely, the plate precipitation device comprises:

an inlet for the particle-containing gas stream and frontally an outletfor the gas stream from which particles have been removed,

a housing provided with a horizontal upper part and a firsttrough-shaped bottom with an ascending slope in relation to thedirection of flow of the gas stream,

a plurality of equidistant collector plates, substantially of equalheight, cooperating with said trough-shaped bottom 7,

a gas flow passage delimited by the upper edges of the plates, the upperpart of the housing and the lateral walls of the housing,

means arranged at the lower point of said trough-shaped bottom, halfwaybetween two adjacent collector plates, through which the fine particlescaught by the plates are discharged,

a second bottom substantially parallel to said trough-shaped bottom, onwhich the fine particles discharged through said means fall and slidedownwards,

at least one dust discharge port at the lower end of said second bottomassociated with a line connected to said dust discharge port,

a dust discharge channel between the two bottoms of the housing,

a wall separating the gas flow passage and the dust discharge channel.

In another variant, the precipitation device comprises at least oneporous structure, cross-linked foam for example, pierced with channelsthrough which the gas containing the remaining particles circulates witha turbulent flow.

The separation assembly according to the invention can also comprise agas stream suction means.

The plate precipitation device belonging to the combination describedabove can have the form of any device described in the aforementionedpatents and preferably comprises plates of equal height mounted, with anascending slope in relation to the direction of flow of the gas, on thebottom of a housing having a horizontal upper end, which provides apassage of decreasing height for the gas stream. The bottom istrough-shaped and comprises, at the lowest point thereof small-diameterholes through which the fine sand is discharged into a second bottom onwhich it slides prior to being discharged through a port into a lineconnected to a container. Peridoc activation of a vibrator arranged onthe outer surface of the second bottom contributes to discharge of thefine dust.

The precipitation device provided with a porous structure pierced withchannels can be similar to those described in patent applicationsFR-2,777,801 and FR-2,769,517 mentioned here by way of reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the global arrangement of an assembly comprising aseparation chamber and a plate precipitation device according to apreferred embodiment of the present invention.

FIG. 2 is a sectional view of the plate precipitation device alongsection AA of FIG. 1.

FIG. 3 shows another variant wherein the separation device is followedby a separation device with precipitation in a porous structure piercedwith channels.

DETAILED DESCRIPTION

A separation device is shown in the lower part of FIG. 1 and it consistsof a chamber 1 provided with lower parts 2 that taper downwards, anaxial circular gas inlet line 3 and a slot-shaped outlet 4 extendingover the total width of chamber 1 and situated at the upper end of aback wall 5 of chamber 1.

According to this embodiment of the invention, the elevated position ofslot-shaped outlet 4 in relation to the bottom of the chamber is atleast equal to the distance over which the sand grains having a size ofabout 30 microns settle during the time of residence of the gas inchamber 1. The particles of larger size settle higher and a great numberthereof falls into hoppers 2 before they reach back wall 5. However,most of the sand grains of a size equal to or greater than 30 micronshit back wall 5 of the chamber.

The 30-micron value is given here for information only.

It has been experimentally discovered that with a low gas flow velocity,of the order of 1 m/s, in chamber 1, the sand grains do not bounce onwall 5 after hitting it, but they fall along the wall into the hopperwhich is the nearest to the outlet. Experience also shows that the sandfalling along wall 5 is not recaptured and re-entrained by the gas. Thephysical reason therefore is that, on wall 5, the gas only has a viscous(non-turbulent) flow whose velocity is much lower than the settlingvelocity of the sand grains. The sand discharged into hoppers 2 isneither recaptured nor re-entrained because the velocity of collectionof the sand grains is much higher than 1 m/s. The volume of chamber 1 issized so that the velocity of the particle-containing sand issufficiently low for the principle of settlement of the heavierparticles thus described to take place.

A separation device 20 is shown in the upper part of FIG. 1. Device 20has been improved in relation to the precipitation devices described inthe prior art and in particular in the patents mentioned at thebeginning of the description.

It mainly comprises a housing provided with a horizontal upper end 8 andsloping bottoms 7 and 13. Vertical collector plates 6 of substantiallyequal height are mounted on sloping bottom 7. The gas is fed intoseparation device 20 through a circular port 10. The height of gas flowpassage 9 in the separation device decreases between inlet 10 and outlet11. The fine sand grains suspended in the gas entering precipitationdevice 20 through inlet 10 are driven by the fluctuations of theturbulent flow velocity into spaces contained between the plates, wherethey hit and settle on plates 6. This phenomenon has already beenexplained in the prior art mentioned at the beginning of thedescription.

It has been experimentally discovered that the removal efficiencyincreases when the height of flow passage 9 is decreasing in relation tothe direction of propagation of the turbulent gas stream. It has beenobserved that the finer particles settle on the plates located in thevicinity of the precipitation device outlet. Furthermore, afterformation of a layer of fine dust on plates 6, it has been observed thatthe deposit comes off by itself under the action of gravity. The dustthen falls from plates 6 onto trough-shaped bottom 7 of theprecipitation device.

FIG. 2 shows a passage 9 divided into four identical sections. This isnot compulsory. FIG. 2 shows that the shape of plates 6 is suited to thetrough shape of bottom 7. Furthermore, bottom 7, two plates 6 and twowalls form a cell comprising, at the lower central point thereof, asmall-diameter circular hole 12 through which the dust is dischargedinto a section 17 provided with a bottom 13. Said dust slides alongbottom 13, assisted by the intermittent activation of a vibrator 14fastened to said bottom 13. The dust then enters a general dustdischarge line 15 arranged at the bottom of the slope formed by the twobottoms. Dust discharge section 17 is separated from gas flow passage 9by wall 16. The dust can finally be discharged into a container (notshown) connected to discharge line 15.

According to the invention, the gas successively flows through thecombination consisting of separation chamber 1 and plate precipitationdevice 20 with the aid of a suction means 18 arranged downstream fromoutlet 11. Discharge and removal of the sand or of the dust present inthis combination are performed continuously, without interruption of thegas flow and separation of solids from the gas.

By way of example of this invention, the following assembly can bementioned: a separation chamber 1 and a plate precipitation deviceintended for processing of a gas flow of about 11,000 Am³/h at atemperature ranging between 500 and 700° C. in which a foundry sand iscarried along, has the following dimensions: length of separationchamber 1: 4.5 m, height (not including the tapered bottom): 1.6 m,width: 1.8 m. Gas outlet slot 4 is 15-cm high. 90% by weight of the sandare removed in chamber 1. Gas flow passage 9 of the plate precipitationdevice is 3.2 m long, 1.8 m wide, and its height decreases from 1.5 m atthe inlet to 7.6 cm at outlet 11. Collector plates 6 are 32 cm high,with a 7.6-cm spacing between the plates. Dust discharge holes 12 are 13mm in diameter. The thermal sand removal efficiency of the combinationconsisting of the separation chamber and the plate precipitation deviceis of the order of 99.5%. The plate precipitation device alone canremove only about 70% by weight of the thermal sand, hence the synergismbetween a type 1 separation chamber and the plate precipitation device.The physical reason for this synergism is that the large-size sandgrains are removed more efficiently in separation chamber 1, whoseseparation principle can be referred to as << gravity separation >>,whereas the finer particles are removed more efficiently in plateprecipitation device 20.

The performances of the separation chamber can be compared with those ofa conventional horizontal gravity separator. In order to obtain a 100%removal efficiency for 30-micron diameter sand grains, a conventionalhorizontal gravity separator with the same height and width as theseparation chamber described in the example above of the presentinvention must be about 48 m long, which is 11 times as great as the 4.5m length of the separation chamber of the present invention. If it wasdecided that the height, the width and the length of a conventionalhorizontal gravity separator should have the same value, it has beencalculated that these three dimensions would then be 9.4 m.

The performances of the separation chamber can also be compared withthose of inertia separators of complex design which can reach a 90%removal efficiency for 30-micron sand grains, but at the cost of apressure drop that is much higher than the 180 Pa measured in theseparation chamber.

High-performance cyclone collectors can nearly reach a 100% removalefficiency for 30-micron sand grains, but also at the cost of a pressuredrop much above 180 Pa.

The total thermal sand removal efficiency obtained with the combinationaccording to the invention, of the order of 99.5%, is matched by noother known mechanical separator.

In the variant illustrated in FIG. 3, device 21 for separating the finerparticles comprises a tubular line 22 connected to inlet 10 intended forthe gas stream flowing out of chamber 1 and to outlet 26 on which a gassuction system can be mounted. Reference number 23 designates severalblocks of porous material pierced with channels along the principal axisof line 22. The separation device can correspond to the device describedin publication WO-99/19,044 corresponding to application FR-2,769,517.The porosity of the foam is greater than 90%, the size of the cellsranges between 0.5 and 5 mm. It is clear that it is an open-cell foam,i.e. the pores communicate with one another. These blocks are piercedwith channels whose diameter ranges for example between 3 and 100 mm. Itworks as follows the turbulent particle-containing gas stream circulatesin the parallel channels, without any notable pressure drop, and theparticles enter the wall of the channels. A vibratory system can expelthe particles out of the porous material and cause them to move towardsa receptacle 24 which collects the particles in a tank 25.

According to the gas flow rate, line 21 can be more or less long andequipped with one or more porous blocks 23 arranged in series.

What is claimed is:
 1. A process for separation of solid particles of wide grain size distribution carried along by a turbulent, high-temperature gas stream resulting from a thermal sand regeneration process, comprising first removing coarser particles in a separation chamber through gravity, then removing finer particles in a precipitation device.
 2. A process as claimed in claim 1, wherein the stage of removal of the coarser particles carried along by the turbulent gas stream comprises passing said stream into the separation chamber so as to reduce the velocity of flow of said gas stream and to cause the coarser particles to settle under the effect of gravity, then to hit a back wall of said separation chamber and to fall along the wall into a lower part of said separation chamber.
 3. A process as claimed in claim 1, wherein the grain size distribution of the particles ranges between 1 μm and about 100 μm.
 4. A process for separation of solid particles having a grain size distribution ranging between 1 μm and about 100 μm present in a turbulent gas stream, comprising first removing coarser particles in a separation chamber through gravity, then removing finer particles in a precipitation device.
 5. A process as claimed in claim 4, wherein the stage of removal of the coarser particles carried along by the turbulent gas stream comprises passing said stream into the separation chamber so as to reduce the velocity of flow of said gas stream and to cause the coarser particles to settle under the effect of gravity, then to hit a back wall of said separation chamber and to fall along the wall into a lower part of said separation chamber.
 6. An assembly for separation of solid particles of wide grain size distribution present in a turbulent gas stream, comprising: a separation chamber intended to receive a high-temperature turbulent gas stream containing said solid particles in suspension and to remove from said stream larger particles under the effect of gravity, and a precipitation device which receives said stream coming from said separation chamber in order to remove finer particles from said stream, wherein said precipitation device comprises: an inlet for the gas stream, connected to outlet of said separation chamber, and an outlet for the gas stream, a housing provided with a horizontal upper part and with a trough-shaped first bottom with an ascending slope in relation to the direction of flow of the gas stream, a plurality of equidistant collector plates substantially of equal height, cooperating with said trough-shaped bottom, a gas flow passage delimited by upper edges of said collector plates, the upper part of the housing and lateral walls of the housing, means arranged at the lowest point of said trough-shaped bottom, halfway between two adjacent collector plates, through which the finer particles collected by said collector plates are discharged, a second bottom substantially parallel to said trough-shaped bottom, on which the finer particles discharged through said means fall and slide downwards, at least one dust discharge port at the lower end of said second bottom, associated with a line connected to said dust discharge port, a dust discharge channel between the two bottoms of the housing, and a wall separating the gas flow passage and the dust discharge channel.
 7. An assembly as claimed in claim 6, wherein said separation chamber comprises: an inlet port for the stream and an outlet port for said stream in form of a horizontal slot provided in a back wall of said separation chamber extending over a total width of said separation chamber and arranged in an upper part of said back wall, and lower parts that taper at a lower end thereof, wherein the larger particles are collected after being removed from said gas stream.
 8. An assembly for separation of solid particles of wide grain size distribution present in a turbulent gas stream, comprising: a separation chamber intended to receive a high-temperature turbulent gas stream containing said solid particles in suspension and to remove from said stream larger particles under the effect of gravity, and a precipitation device which receives said stream coming from said separation chamber in order to remove finer particles from said stream, wherein the precipitation device comprises at least one block of porous material arranged in a line which communicates with an outlet of said separation chamber, said block being pierced with channels in the direction of the principal axis of said line so that the gas stream circulates through said channels.
 9. An assembly as claimed in claim 8, wherein the precipitation device comprises several blocks of porous material arranged in series and spaced out along said line.
 10. An assembly as claimed in claim 8, wherein said precipitation device also comprises a gas stream suction means.
 11. An assembly as claimed in claim 8, wherein said separation chamber comprises: an inlet port for the stream and an outlet port for said stream in form of a horizontal slot provided in a back wall of said separation chamber extending over a total width of said separation chamber and arranged in an upper part of said back wall, and lower parts that taper at a lower end thereof, wherein the larger particles are collected after being removed from said gas stream. 